Design Meeting Notes: January 8, 2015

Multi-level Include syntax

Since EF 4.1, multi-level includes have been possible using either string syntax:

customers.Include("Contact.HomeAddress");
customers.Include("Orders.OrderDetails")

or lambda syntax:

customers.Include(c => c.Contact.HomeAddress);
customers.Include(c => c.Orders.Select(o => o.OrderDetails));

Note that for the lambda syntax we make use of the LINQ Select method when adding an additional level of include after a collection navigation property. This is because the collection navigation property is an IEnumerable<> of the entity type rather than the entity type itself.

For example, to include all products and their suppliers, plus all reviews and review authors:

categories
    .Include(c => c.Products.Select(p => p.Reviews.Select(r => r.Author)))
    .Include(c => c.Products.Select(p => p.Supplier));

The Select syntax has two issues:

• It is not very discoverable
• It reuses the LINQ Select operator for something that could be argued is not selecting

The second issue could be addressed with a new method, but the discoverability issue would still remain.

Proposal

A new proposal is to use a pattern similar to LINQ OrderBy:

customers.Include(c => c.Contact).ThenInclude(c => c.Address);
customers.Include(c => c.Orders).ThenInclude(o => o.OrderDetails);

This means that:

• Lambdas can be valid dotted property expressions
• It works the same for references and collections

The example from above would now look like this:

categories
    .Include(c => c.Products).ThenInclude(p => p.Reviews).ThenInclude(r => r.Author)
    .Include(c => c.Products).ThenInclude(p => p.Supplier);

Please use the design meeting discussion issue to provide feedback, ask questions, etc.